A zinc/air cell is a type of metal/air cell depolarized with air. Zinc/air cells can also be considered as a type of alkaline cell, since aqueous alkaline electrolyte is normally added to the zinc anode. Zinc/air cells are typically in the form of button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each can have a closed end and an open end. An electrical insulating material can be placed around the outside surface of the anode casing. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. The anode mixture for zinc/air cells, normally contains zinc particles amalgamated with mercury (typically the mercury is about 3 percent by weight of the anode). The anode mixture also contains a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used.
The cathode casing (cathode cup) contains an air diffuser (air filter) which lines the inside surface of the cathode casing's closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent to air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of particulate manganese dioxide, conductive carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. An electrically conductive carbon material is typically added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing.
The cathode casing can typically be of nickel plated cold rolled steel or nickel clad stainless steel with the nickel layer preferably on both inner and outer surfaces of the cold rolled or stainless steel. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing's outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell's negative terminal and a portion of the closed end of the cathode casing can function as the cell's positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to reduce the hydrogen gassing which can occur as a side reaction in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The mercury, for example, reduces the rate of the zinc corrosion side reaction involving reaction of zinc with water producing zinc oxide and hydrogen gas. The gassing, if excessive, can reduce the cell capacity (mAmp-hrs) and increase the chance of electrolyte leakage. Such leakage can damage or destroy the hearing aid or other electronic component being powered. The mercury also improves electrical conductivity between the zinc particles. Many regions around the world now restrict the use of mercury in electrochemical cells because of environmental concerns. Although mercury has generally been eliminated from conventional zinc/MnO2 alkaline cells, it is still employed in many zinc/air button cells because such cells are usually very small and thus contain only very small total amount of mercury. However, in view of increasing regulation against use of mercury, it is desirable to reduce or eliminate added mercury from zinc/air button cells as well.
Zinc anodes for zinc/air cells may be prepared in the form of a slurry which is pumped into the cell's anode cavity with the aid of a slurry pump. The slurry may comprise a gelled mixture of amalgamated zinc particles, gelling agent such as polyacrylic acid, and aqueous alkaline electrolyte solution. However, it is usually desired to have a high zinc content for the zinc/air cell anode, for example, between about 75 to 80 percent or higher. In such case the anode mixture can becomes too heavy for pumping. Instead the anode is typically prepared by dispensing a mixture of amalgamated zinc particles and gelling agent into cell's anode cavity and the adding aqueous alkaline electrolyte to form the gelled anode. In commercial production this becomes an inefficient and speed limited method of forming the anode for zinc/air cells. It can also lead to nonuniform dispersion of zinc particles within the gelled anode. Also, some settling or precipitation of zinc particles within the gelled anode can occur, for example, in cases where the cells are stored for long time or experience some shock or vibration.
If the zinc content is reduced, a pumpable anode slurry can be prepared. The anode slurry can be prepared by forming a gelled electrolyte mixture comprising an aqueous alkaline electrolyte, preferably a gelled aqueous potassium hydroxide. Such gelled electrolyte can be formed, for example, by mixing a gelling agent such as a polyacrylic acid gelling agent with aqueous alkaline electrolyte. A dry zinc powder, typically amalgamated with mercury, is then mixed into the gelled electrolyte to form an anode slurry mixture. The anode slurry mixture is conventionally pumped into the zinc/air cell's anode cavity by a slurry pump. Although the slurry pump is designed to keep air from entering the slurry mixture, the pump nevertheless typically allows small amounts of air to enter the slurry as it is pumped into the cell's anode cavity. This causes small air pockets to develop within the anode mixture pumped into the cell's anode cavity. Such air pockets can be microscopic in size but more typically there are a number of such air pockets which are visible to the naked eye when the anode is examined by simple X-ray photography without magnification. Optionally some additional aqueous electrolyte may be added to adjust the electrolyte composition in the anode mixture after the slurry is pumped into the anode cavity. However, such additional electrolyte does not noticeably reduce the number of air pockets within the anode slurry. The presence of such air pockets within the anode slurry tends to reduce the overall conductivity of the anode because it reduces the level of zinc interparticle contact in the region of the air pockets.
In a commercial cell assembly line such zinc slurry tends to plug the dispensing tube, leading to down time of the assembly line. Also, the storage of large batches of the slurry tends to result in settling or precipitation of some of the zinc particles over time, which can result ultimately in a non-uniform distribution of zinc particles. The zinc particle settling or precipitation could also occur from the gelled zinc slurry already dispensed into the cells, for example, in cases where the cells are stored for long time or experience some shock or vibration. Such precipitation of the zinc particles will cause non-uniform distribution of the zinc within the anode cavity and subsequent loss of electrical continuity. Also pumping the anode slurry into very small dimensioned, particularly, small irregular shaped anode cavities becomes difficult. In such case the slurry mixture must be prepared with more attention given to zinc particle shape and slurry flowability (consistency) which can compromise the final anode from the standpoint of overall conductivity.
It is desirable to eliminate the need for either dispensing an anode slurry into the anode cavity or alternatively dispensing a mixture of zinc particles and gelling agent into the anode cavity and then adding aqueous electrolyte. Rather, it is desired to prepare the anode in preformed solid form which can be inserted as a dry solid into the anode cavity without the need of pumping a slurry mixture into the anode cavity. Thus, it is desired to eliminate the need for pumping the anode mixture into the anode cavity with a slurry pump or other pumping device.